I. Field of the Invention
The present invention relates to a dynamic pressure air bearing and, more particularly, to a dynamic pressure air bearing for use in a device, such as a laser scanner, having a high-precision rotating mechanism which rotates at high speed.
II. Prior Art
A laser scanner employs a mechanism which includes a rotating member which must be able to rotate at high speed (e.g., 10,000 rpm) and with high precision. The scanner, therefore, requires a bearing which can support the rotating member, allowing the rotating member to rotate at high speed with high precision for extended periods of time.
Ball bearings were used in the laser scanner. They require lubricant. The lubricant is scattered around when the rotating member rotates at such a high speed. Hence, ball bearings cannot be used in a laser scanner whose operation can be impaired by scattered lubricant. Further, ball bearings cannot allow the rotating member to rotate at speed as high as 10,000 rpm for an extended period of time, due to the inevitable wear of the balls of the bearings.
Dynamic pressure air bearings are often used in the high-speed rotation mechanism, to enable the rotating member to rotate at high speed for extended periods of time. A dynamic pressure air bearing comprises a fixed shaft with a surrounding sleeve. A groove is cut in the peripheral surface of the shaft. This groove extends obliquely to the axis of the shaft. When the sleeve is rotated at a high rated speed, air flows through the groove. The air serves as a lubricant because of its dynamic pressure. Hence, the sleeve can rotate without coming to contact with the fixed shaft. However, when the sleeve is rotating at a lower speed, such as at the start or at the end of rotation, the sleeve and the shaft rub against each other, and become worn. Therefore, the surface of the dynamic pressure portion of the shaft and the inner surface of the sleeve must be covered with a layer of harded stainless steel, superhard alloy or high-speed steel, or the like.
However, these hard metals are very expensive and also very hard to work. Further, with the dynamic pressure air bearing, there must be a clearance of several microns between the sleeve and the dynamic pressure portion of the shaft. Obviously, the bearing must be machined with high precision to have the designed structure. In other words, its parts must be accurately machined to the desired roundness and straightness. When the parts are made of hard metals of low workability, however, they cannot easily be machined to the required accuracy. The conventional dynamic pressure air bearing can, therefore, hardly be mass-produced. A dynamic pressure air bearing comprising a rotating shaft and a fixed sleeve cannot be mass-produced for the same reason, either.